Trucks and tractor-trailer combinations frequently transport cargo that must be maintained at a predetermined temperature during transportation. Vehicles that transport temperature sensitive cargo typically have one or more load spaces that are maintained at a set point temperature by a temperature control unit, having a compressor, a condenser, a flow control valve, an expansion valve, and an evaporator coil. Operation of the temperature control unit is generally controlled and monitored by an electronic controller.
Generally, temperature control units operate in cooling and heating modes, depending, at least in part, upon the temperature of the load space and the ambient temperature outside the air-conditioned space. When the temperature of the load space is above the set point temperature, the temperature control units operate in the cooling mode to pull down the temperature in the load space. During operation in the cooling mode, refrigerant is directed along a refrigerant circuit, which extends between the compressor, the flow control valve, the condenser, the expansion valve, and the evaporator coil. Load space air is then exposed to the relatively cool evaporator coil.
When the temperature of the load space is below the set point temperature, the temperature control units operate in a heating mode. During operation in the heating mode, relatively warm refrigerant is directed through a heating circuit, which extends from the compressor, the flow control valve, and the evaporator coil. Load space air is then exposed to the relatively warm evaporator coil.
Additionally, to minimize the formation of ice and/or frost on the evaporator coil and to ensure that the temperature control unit is operating in the most efficient manner, temperature control units periodically operate in a defrost mode. During operation in the defrost mode, relatively warm refrigerant is directed through the heating circuit to defrost the evaporator coil. Additionally, during operation in the defrost mode, interaction between the load space air and the heating circuit is limited.